The specific aims of this protocol are to classify respiratory events (apnea and hypopnea) and periodic breathing in critically ill patients as either central or obstructive using a nasal cannula/pressure transducer system allowing recognition of flow limitation; to compare this classification to the reference standard measure of respiratory effort by esophageal manometry; to characterize the respiratory events detected in the ICU/post-operative patients who are not suspected of having a primary sleep disorder; and to correlate the inpatient sleep patterns with a full night sleep study.